Hybrid apu start fuel system

ABSTRACT

A fuel system for an auxiliary power unit includes a mechanical fuel pump, an electric fuel pump, and a controller. The mechanical fuel pump provides fuel flow to the auxiliary power unit and has an output dependent upon an operational speed of the auxiliary power unit. The electric fuel pump provides fuel flow to the auxiliary power unit, and is located in flow series with the mechanical fuel pump. The controller causes the electric fuel pump to provide fuel flow during starting of the auxiliary power unit.

BACKGROUND

The present disclosure relates generally to auxiliary power units, andmore particularly to fuel systems for providing fuel during starting ofauxiliary power units.

Auxiliary power units (APUs) are used on aircraft for main enginestarting, to provide in-flight engine restart assistance, and to supplypower and compressed air for ground and in-flight operations. APUs aretypically gas turbine engines that include a compression section, acombustion section, and a turbine section. The compression section drawsin and compresses ambient air. The compressed air is mixed with fuel andignited in the combustion section to produce high-energy combusted air.The combusted air is expanded in the turbine section, which rotates todrive the compression section and to provide additional power through anoutput shaft. Typically a gearbox is mounted to the output shaft of APUsfor driving accessory generators and various pumps. Fuel is sent to thecombustion section of an APU by one or more fuel pumps. Commonly usedfuel pumps are high pressure, fixed volume, gear-driven type pumps thatrequire mechanical rotation of the engine to produce fuel flow. During astart up operation of the APU, gear-driven pumps rotate at speeds muchlower than normal operating speeds. In order to deliver the requiredvolume of fuel for initial ignition in the combustor, the gear-drivenpump must be sized to have a large displacement. Once the APU is startedand rotating at normal operating speeds, the displacement of thegear-driven pump provides an excess volume of fuel flow. Accordingly,the typical gear-driven pump is over-sized and provides capacity inexcess of the needs of normal APU operation thereby adding unnecessarysize and weight.

SUMMARY

A fuel system for an auxiliary power unit includes a mechanical fuelpump, an electric fuel pump, and a controller. The mechanical fuel pumpprovides fuel flow to the auxiliary power unit and has an outputdependent upon an operational speed of the auxiliary power unit. Theelectric fuel pump provides fuel flow to the auxiliary power unit and islocated in flow series with the mechanical fuel pump. The controllercauses the electric fuel pump to provide fuel flow during starting ofthe auxiliary power unit.

A fuel system for an auxiliary power unit includes a fuel supplyconduit, a fuel metering unit, a mechanical gear driven pump, amechanical boost pump, an electric pump, and a controller. The fuelsupply conduit fluidly connects a fuel reservoir to a combustor. Thefuel metering unit is located on the fuel supply conduit upstream of thecombustor. The mechanical gear driven pump is located on the fuel supplyconduit upstream of the fuel metering unit. The mechanical boost pump islocated on the fuel supply conduit upstream of the mechanical geardriven pump. The electric pump is located on the fuel supply conduitupstream of the mechanical gear driven pump. The controller commands theelectric pump to flow fuel through the mechanical gear driven pump tothe fuel metering unit during starting of the auxiliary power unit.

A method of starting an auxiliary power unit includes providing power toa starter motor for starting rotation of the auxiliary power unit andproviding power to an electric pump for flowing fuel to the auxiliarypower unit. The step of providing power to the starter motor and thestep of providing power to the electric pump occur simultaneously.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a fuel system in accordance with theprior art.

FIG. 2 is a schematic diagram of a first embodiment of a fuel systemhaving an electric pump in accordance with the present disclosure.

FIG. 3 is a schematic diagram of a second embodiment of a fuel systemhaving an electric pump in accordance with the present disclosure.

FIG. 4 is a schematic diagram showing a mechanical drive and powersupply for fuel systems in accordance with the present disclosure.

FIG. 5 is a schematic diagram showing a shut-off valve and relief fuelconduit for fuel systems in accordance with the present disclosure.

DETAILED DESCRIPTION

FIG. 1 is a schematic diagram of fuel system 10 for an auxiliary powerunit (APU) in accordance with the prior art. Fuel system 10 includesfuel supply conduit 12, fuel supply 14, mechanical gear-driven pump 16,fuel metering unit 18, engine 20, controller 22, and fuel recirculationconduit 24. Fuel system 10 provides fuel from fuel supply 14 to engine20 for combustion.

Fuel supply conduit 12 fluidly connects fuel supply 14 to a combustor inengine 20. More specifically, fuel supply conduit 12 connects fuelsupply 14, mechanical gear-driven fuel pump 16, fuel metering unit 18,and engine 20 in flow series. Fuel metering unit 18 is located upstreamof engine 20, mechanical gear-driven fuel pump 16 is located upstream offuel metering unit 18, and fuel supply 14 is located upstream ofmechanical gear-driven fuel pump 16 along fuel supply conduit 12.Controller 22 is electrically connected to fuel metering unit 18. Fuelrecirculation conduit 24 fluidly connects fuel metering unit 18 to fuelsupply conduit 12 near mechanical gear-driven fuel pump 16. Morespecifically, an inlet of fuel recirculation conduit 24 is connected tofuel metering unit 18 and an outlet of fuel recirculation conduit 24 isconnected to fuel supply conduit 12 at a location downstream of fuelsupply 14 and a location upstream of mechanical gear-driven fuel pump16.

During use of engine 20, fuel flows from fuel supply 14 to engine 20along fuel supply conduit 12. More specifically, fuel flows from fuelsupply 14 along fuel supply conduit 12 to mechanical gear-driven fuelpump 16. Mechanical gear-driven fuel pump 16 pumps fuel along fuelsupply conduit 12 to fuel metering unit 18. Fuel metering unit 18receives a signal from controller 22 that indicates desired fuel flow toengine 20 based on operational need. Fuel metering unit 18 responds tothe signal from controller 22 by then providing the desired fuel flow toengine 20 via fuel supply conduit 12. Fuel in excess of the amountrequired by engine 20 is sent from fuel metering unit 18 back to fuelsupply conduit 12 along fuel recirculation conduit 24.

The fuel output of mechanical gear-driven fuel pump 16 is dependent onthe operating speed of engine 20. When engine 20 is operating at normal,self-sustaining speeds (e.g. 50-100% of “maximum rated speed”, i.e.maximum rotation speed at which engine will operate safely), mechanicalgear-driven pump 16 is relatively efficient and provides more thanenough fuel flow to fuel metering unit 18. At lower operating speeds(e.g. 1-10% of maximum rated speed), mechanical gear-driven pump 16 isfar less efficient and may not provide sufficient fuel flow. Thisproblem is particularly pronounced during starting, when engine 20operating speed is at or below about 12% of rated speed. It is, however,desirable to start engine 20 at a low rotational speed to facilitateignition in the combustor. As a consequence, mechanical gear-driven pump16 is oversized to meet the needs of starting fuel flow requirementsrelative to the size needed at higher engine operating speeds. Thisover-sizing of mechanical gear-driven pump 16 increases weight and powerneeds, as well as results in excessive fuel flow and waste heat.

The present disclosure is directed to improving the ability of fuelsystems to deliver fuel of sufficient quantity and pressure to achievefuel ignition during low speed engine starting. As described in detailbelow with reference to FIGS. 2-5, the disclosure suggests improving thelow speed efficiency of mechanical gear-driven fuel pump (such asmechanical gear-driven fuel pump 16 of FIG. 1) by adding an electricmotor-driven fuel pump in series. The electric fuel pump can be drivenindependent of engine operating speed to supplement fuel flow when themechanical gear-driven pump is operating at a low rotational speed (e.g.during APU starting). The proposed electric motor-driven fuel pumpincreases fuel flow/pressure to the mechanical gear-driven pump, therebysupercharging an inlet of the mechanical gear-driven pump.

FIG. 2 is a schematic diagram of fuel system 26 having electric fuelpump 28 for an APU in accordance with a first embodiment of the presentdisclosure. Fuel system 26 includes electric fuel pump 28, fuel supplyconduit 30, fuel supply 32, mechanical boost fuel pump 34, mechanicalgear-driven fuel pump 36, fuel metering unit 38, engine 40, controller42, electrical connections 44, fuel recirculation conduit 46 havinginlet 48 and outlet 50, fuel bypass conduit 52 having inlet 54 andoutlet 56, bypass check valve 58, and rotational shaft 60. Fuel system26 is presented in a simplified form and can additionally includefilters, heat exchangers, and/or other conventional components known inthe art. Inclusion of electric pump 28 allows fuel system 26 to morefuel at low engine 40 speed than would be otherwise possible.

Fuel supply conduit 30 fluidly connects fuel supply 32 to a combustor inengine 40. More specifically, fuel supply conduit 30 connects fuelsupply 32, electric fuel pump 28, mechanical boost fuel pump 34,mechanical gear-driven fuel pump 36, fuel metering unit 38, and engine40 in flow series. Along fuel supply conduit 30, fuel metering unit 38is located upstream of engine 40, mechanical gear-driven fuel pump 36 islocated upstream of fuel metering unit 38, mechanical boost fuel pump 34is located upstream of mechanical gear-driven fuel pump 36, electricfuel pump 28 is located upstream of mechanical boost fuel pump 34, andfuel supply 32 is located upstream of electric fuel pump 28. Controller42 is electrically connected to fuel metering unit 38 and to electricfuel pump 28 by electrical connections 44.

Fuel recirculation conduit 46 fluidly connects fuel metering unit 38 tofuel supply conduit 30 at or near mechanical gear-driven pump 36. Morespecifically, inlet 48 of fuel recirculation conduit 46 is connected tofuel metering unit 38 and outlet 50 of fuel recirculation conduit 46 isconnected to fuel supply conduit 30 at a location downstream ofmechanical boost fuel pump 34 and a location upstream of mechanicalgear-driven fuel pump 36. Fuel bypass conduit 52 fluidly connects fuelsupply conduit 30 upstream of electric fuel pump 28 to fuel supplyconduit 30 downstream of electric fuel pump 28. More specifically, inlet54 of fuel bypass conduit 52 is connected to fuel supply conduit 30 at alocation downstream of fuel supply 32 and a location upstream ofelectric fuel pump 28, and outlet 56 of fuel bypass conduit 52 isconnected to fuel supply conduit 30 at a location downstream of electricfuel pump 28 and a location upstream of mechanical boost fuel pump 34.Bypass check valve 58 is located on bypass fuel conduit 52. Rotationalshaft 60 connects mechanical boost fuel pump 34 to mechanicalgear-driven fuel pump 36.

For low speed operation of engine 40 (e.g. engine starting), controller42 commands electric fuel pump 28 to operate via electrical connection44. Fuel flows from fuel supply 32 along fuel supply conduit 30 toelectric fuel pump 28, which pumps fuel along fuel supply conduit 30 toa combustor in engine 40 (through mechanical boost fuel pump 34,mechanical gear-driven fuel pump 36, and fuel metering unit 38). Sinceelectric fuel pump 28 is driven independently of engine 40 (e.g. by amotor), fuel flow output of sufficient pressure and quantity isimmediately available for engine 40 at low engine speed. For example,sufficient fuel for combustor ignition is anticipated at around 3% ofmax engine speed. In comparison, the prior art fuel system of FIG. 1which relies solely on mechanical gear-driven fuel pump 16, willtypically require a minimum of about 8% of engine max speed to developsufficient fuel flow for combustor ignition, Accordingly, by inclusionof electric fuel pump 28 in fuel system 26, the window for combustorlight-off (i.e. engine speed range in which ignition of fuel ispossible) is lengthened from about 8-15% of max engine speed to about3-15% of max engine speed by inclusion of electric fuel pump 28 in fuelsystem 26.

In addition to electric fuel pump 28, mechanical boost fuel pump 34 andmechanical gear-driven boost pump 36 pump fuel along fuel supply conduit30 to engine 40. Electric fuel pump 28, mechanical boost fuel pump 34and mechanical gear-driven fuel pump 36 are located in flow series. Fuelflows to mechanical boost fuel pump 34 from electric fuel pump 28 viafuel supply conduit 30. Mechanical boost fuel pump 34 pumps fuel alongfuel supply conduit 30 to mechanical gear-driven fuel pump 36, whichpumps fuel along fuel supply conduit 30 to fuel metering unit 38. Fuelmetering unit 38 receives a signal from controller 42 that indicatesproper fuel flow to engine 40 based on operational need. Fuel meteringunit 38 responds to the command from controller 42 via electricalconnection 44 by providing the proper fuel flow to engine 40 via fuelsupply conduit 30. Fuel metering unit 38 can include a fuel meteringvalve, a differential pressure (delta P) valve, and/or a solenoid valveas known in the art. Fuel in excess of the amount required by engine 40is sent from fuel metering unit 38 back to fuel supply conduit 30 alongfuel recirculation conduit 46. More specifically, excess fuel exits fuelmetering unit 38, flows through inlet 48 of fuel recirculation conduit46 to outlet 50 and back into fuel supply conduit 30 at a location nearan inlet of mechanical gear-driven fuel pump 36.

Fuel flow output of both mechanical boost fuel pump 34 and mechanicalgear-driven fuel pump 36 is dependent upon and proportional to theoperating speed of engine 40. Accordingly, mechanical boost fuel pump 34and mechanical gear-driven fuel pump 36 are inefficient at low speedsand increasingly efficient as engine 40 speed increases. Once engine 40reaches about 50% of max speed, mechanical boost fuel pump 34 andmechanical gear-driven fuel pump 36 provide more than enough fuel flowfor the needs of engine 40 (i.e. engine is self-sustaining). Controller42 then commands electric fuel pump 28 to cease operation, therebyextending the life of electric fuel pump 28 and saving energy. At oraround the same time, bypass check valve 58 on bypass fuel conduit 52will change from a closed state to an open state, thereby allowing fuelto flow along bypass fuel conduit 52 and around electric fuel pump 28.

In the depicted embodiment, bypass check valve 58 is a passive pressurevalve that remains closed when the pressure differential across bypasscheck valve 58 is positive, (pressure in conduit 52 is greater than inconduit 54) and opens when a pressure differential across bypass checkvalve 58 is negative, (pressure in conduit 52 is less than in conduit54) (e.g. when electric pump 28 is turned off). When the pressuredifferential across bypass check valve 58 causes bypass check valve 58to open, fuel will flow from fuel supply 32 along fuel supply conduit 30to inlet 54 of bypass fuel conduit 52, through bypass check valve 58, tooutlet 56 of bypass fuel conduit 52, and back into fuel supply conduit30 at a location downstream of electric pump 28 and upstream ofmechanical boost fuel pump 34. In other words, fuel will bypass electricpump 28 when bypass check valve 58 is open. Fuel flow along bypass fuelconduit 52 can alternatively be managed by other types of valves (e.g.active valves controlled by controller 42 in coordination with otherevents in fuel system 26). In another embodiment, fuel flows alongbypass fuel conduit 52 once electric fuel pump 28 is turned off and/orwhen fuel flow exceeds a flow capacity of the electric fuel pump 28.Fuel system 26 provides electric fuel pump 28 for supplementing fuelflow during low speed operation of engine 40. When electric fuel pump 28is no longer needed (i.e. higher speed operation of engine 40), fuelbypasses electric fuel pump 28 and is pumped solely by mechanical boostfuel pump 34 and mechanical gear-driven pump 36.

FIG. 3 is a schematic diagram of fuel system 62 having electric fuelpump 28 for an APU in accordance with a second embodiment of the presentdisclosure. Fuel system 62 includes electric fuel pump 28, fuel supplyconduit 30, fuel supply 32, mechanical boost fuel pump 34, mechanicalgear-driven fuel pump 36, fuel metering unit 38, engine 40, controller42, electrical connections 44, fuel recirculation conduit 46 havinginlet 48 and outlet 50, fuel bypass conduit 52 having inlet 54 andoutlet 56, bypass check valve 58, and rotational shaft 60. Fuel system62 is presented in a simplified form and can additionally includefilters, heat exchangers, and/or other conventional components known inthe art. Inclusion of electric fuel pump 28 allows fuel system 62 toprovide more fuel at low engine 40 speed than would be otherwisepossible.

Fuel system 62 of FIG. 3 is similar to fuel system 26 of FIG. 2 and likenumerals indicate like components. Fuel system 62 of FIG. 3 differs fromfuel system 26 of FIG. 2 in that electric fuel pump 28 is locatedbetween mechanical boost fuel pump 34 and mechanical gear-driven fuelpump 36. Despite the alternative placement for electric fuel pump 28,fuel system 62 of FIG. 3 is operated in a manner similar to fuel system26 of FIG. 2. The following discussion highlights the similarities anddifferences between fuel system 62 of FIG. 3 and fuel system 26 of FIG.2.

In fuel system 62, fuel supply conduit 30 fluidly connects fuel supply32 to a combustor in engine 40. More specifically, fuel supply conduit30 connects fuel supply 32, mechanical boost fuel pump 34, electric fuelpump 28, mechanical gear-driven fuel pump 36, fuel metering unit 38, andengine 40 in flow series. Along fuel supply conduit 30, fuel meteringunit 38 is located upstream of engine 40, mechanical gear-driven fuelpump 36 is located upstream of fuel metering unit 38, electric fuel pump28 is located upstream of mechanical gear-driven fuel pump 36,mechanical boost fuel pump 34 is located upstream of electric fuel pump28, and fuel supply 32 is located upstream of mechanical boost fuel pump34. Controller 42 is electrically connected to fuel metering unit 38 andto electric fuel pump 28 by electrical connections 44.

Fuel recirculation conduit 46 fluidly connects fuel metering unit 38 tofuel supply conduit 30. More specifically, inlet 48 of fuelrecirculation conduit 46 is connected to fuel metering unit 38 andoutlet 50 of fuel recirculation conduit 46 is connected to fuel supplyconduit 30 at a location downstream of electric fuel pump 28 and alocation upstream of mechanical gear-driven fuel pump 36. Fuel bypassconduit 52 fluidly connects fuel supply conduit 30 upstream of electricfuel pump 28 to fuel supply conduit 30 downstream of electric fuel pump28. More specifically, inlet 54 of fuel bypass conduit 52 is connectedto fuel supply conduit 30 at a location downstream of mechanical boostfuel pump 34 and a location upstream of electric pump, and outlet 56 offuel bypass conduit 52 is connected to fuel supply conduit 30 at alocation downstream of electric fuel pump 28 and a location upstream ofmechanical gear-driven fuel pump 36. Bypass check valve 58 is located onbypass fuel conduit 52. Rotational shaft 60 connects mechanical boostfuel pump 34 to mechanical gear-driven fuel pump 36.

Mechanical boost fuel pump 34, electric fuel pump 28, and mechanicalgear-driven fuel pump 36 are located in flow series along fuel supplyconduit 30. Fuel flows from fuel supply 32 along fuel supply conduit 30to mechanical boost fuel pump 34. Fuel is pumped to electric fuel pump28 from mechanical boost pump 34 through fuel supply conduit 30.Electric fuel pump 28 pumps fuel received from mechanical boost fuelpump 34 along fuel supply conduit 30 to mechanical gear-driven fuel pump36, which pumps fuel along fuel supply conduit 30 to fuel metering unit38. Fuel metering unit 38 receives a signal from controller 42 viaelectrical connection 44 that indicates proper fuel flow to engine 40based on operational need. Fuel metering unit 38 responds to the commandfrom controller 42 by providing the proper fuel flow to engine 40 viafuel supply conduit 30. Fuel metering unit 38 can include a fuelmetering valve, a differential pressure (delta P) valve, and/or asolenoid valve as known in the art. Fuel in excess of the amountrequired by engine 40 is sent from fuel metering unit 38 back to fuelsupply conduit 30 along fuel recirculation conduit 46. Morespecifically, excess fuel exits fuel metering unit 38, flows throughinlet 48 of fuel recirculation conduit 46 to outlet 50 and back intofuel supply conduit 30 at a location downstream of electric fuel pump 28and upstream of mechanical gear-driven fuel pump 36.

Fuel flow output of both mechanical boost fuel pump 34 and mechanicalgear-driven fuel pump 36 is dependent upon and proportional to theoperating speed of engine 40. Accordingly, mechanical boost fuel pump 34and mechanical gear-driven fuel pump 36 are inefficient at low speedsand increasingly efficient as engine 40 speed increases. At low engineoperating speed, controller 42 commands electric fuel pump 28 to operatevia electrical connection 44 and supplement the inefficient pumping ofmechanical boost fuel pump 34 and mechanical gear-driven fuel pump 36.Since electric fuel pump 28 is driven independently of engine 40 (e.g.by a motor), fuel flow output of sufficient pressure and quantity isimmediately available for engine 40 at very low engine speed. Onceengine 40 reaches about 50% of max speed, mechanical boost fuel pump 34and mechanical gear-driven fuel pump 36 provide more than enough fuelflow for the needs of engine 40 (i.e. engine is self-sustaining).Controller 42 then commands electric fuel pump 28 to cease operation,thereby extending the life of electric fuel pump 28 and saving energy.At or around the same time, bypass check valve 58 on bypass fuel conduit52 will change from a closed state to an open state, thereby allowingfuel to flow along bypass fuel conduit 52.

In the depicted embodiment, bypass check valve 58 is a passive pressurevalve that remains closed when the pressure differential across bypasscheck valve 58 is positive, (pressure in conduit 52 is greater than inconduit 54) and opens when a pressure differential across bypass checkvalve 58 is negative, (pressure in conduit 52 is less than in conduit54) (e.g. when electric pump 28 is turned off and not providing assist).When the pressure differential across bypass check valve 58 causesbypass check valve 58 to open, fuel will flow from mechanical boost fuelpump 34 along fuel supply conduit 30 to inlet 54 of bypass fuel conduit52, through bypass check valve 58, to outlet 56 of bypass fuel conduit52, and back into fuel supply conduit 30 at a location downstream ofelectric fuel pump 28 and upstream of mechanical gear-driven fuel pump36. In other words, fuel will bypass electric fuel pump 28 when bypasscheck valve 58 is open. In another embodiment, fuel flows along bypassfuel conduit 52 once electric fuel pump 28 is turned off and/or whenfuel flow exceeds a flow capacity of the electric fuel pump 28. Fuelsystem 26 provides electric fuel pump 28 for supplementing fuel flowduring low speed operation of engine 40. When electric fuel pump 28 isno longer needed (i.e. higher speed operation of engine 40), fuelbypasses electric fuel pump 28 and is pumped solely by mechanical boostfuel pump 34 and mechanical gear-driven pump 36.

FIG. 4 is a schematic diagram of fuel system 64 having mechanical drive66 and power supply 68 for an APU. Fuel system 64 includes electric fuelpump 28, fuel supply conduit 30, fuel supply 32, mechanical boost fuelpump 34, mechanical gear-driven fuel pump 36, fuel metering unit 38,engine 40, controller 42, electrical connections 44A-44C, recirculationfuel conduit 46 having inlet 48 and outlet 50, bypass fuel conduit 52having inlet 54 and outlet 56, bypass check valve 58, rotational shaft60, mechanical drive 66, power supply 68, starter relay 70, startermotor 72, and power connections 74A-74D. Fuel system 64 is presented ina simplified form and can additionally include filters, heat exchangers,and/or other conventional components known in the art. Inclusion ofelectric fuel pump 28 allows fuel system 64 to provide fuel flowindependent of engine 40 operating speed.

Fuel system 64 of FIG. 4 is similar to fuel system 62 of FIG. 3 and likenumerals indicate like components. The components (e.g. electric fuelpump 28, fuel supply conduit 30, fuel supply 32, mechanical boost fuelpump 34, mechanical gear-driven fuel pump 36, fuel metering unit 38,engine 40, controller 42, electrical connections 44, fuel recirculationconduit 46 having inlet 48 and outlet 50, fuel bypass conduit 52 havinginlet 54 and outlet 56, bypass check valve 58, rotational shaft 60) andstructure of fuel system 64 of FIG. 4 are similar to those describedabove with reference to fuel system 62 of FIG. 3. Fuel system 64 of FIG.4 differs from fuel system 62 of FIG. 3 in that mechanical drive 66 andpower supply 68 are illustrated to describe a method of operating fuelsystem 64. The following discussion highlights mechanical and electricaloperation of fuel system 64 of FIG. 4, which is applicable to any fuelsystem having both mechanical and electric fuel pumps (such as fuelsystem 62 of FIG. 3 and fuel system 26 of FIG. 2).

In fuel system 64, fuel supply conduit 30 fluidly connects fuel supply32 to a combustor in engine 40. More specifically, fuel supply conduit30 connects fuel supply 32, mechanical boost fuel pump 34, electric fuelpump 28, mechanical gear-driven fuel pump 36, fuel metering unit 38, andengine 40 in flow series. Along fuel supply conduit 30, fuel meteringunit 38 is located upstream of engine 40, mechanical gear-driven fuelpump 36 is located upstream of fuel metering unit 38, electric fuel pump28 is located upstream of mechanical gear-driven fuel pump 36,mechanical boost fuel pump 34 is located upstream of electric fuel pump28, and fuel supply 32 is located upstream of mechanical boost fuel pump34.

Fuel recirculation conduit 46 fluidly connects fuel metering unit 38 tofuel supply conduit 30 near mechanical gear-driven fuel pump 36. Morespecifically, inlet 48 of fuel recirculation conduit 46 is connected tofuel metering unit 38 and outlet 50 of fuel recirculation conduit 46 isconnected to fuel supply conduit 30 at a location downstream of electricfuel pump 28 and a location upstream of mechanical gear-driven fuel pump36. Fuel bypass conduit 52 fluidly connects fuel supply conduit 30upstream of electric fuel pump 28 to fuel supply conduit 30 downstreamof electric fuel pump 28. More specifically, inlet 54 of fuel bypassconduit 52 is connected to fuel supply conduit 30 at a locationdownstream of fuel supply 32 and a location upstream of electric pump,and outlet 56 of fuel bypass conduit 52 is connected to fuel supplyconduit 30 at a location downstream of electric fuel pump 28 and alocation upstream of mechanical boost fuel pump 34. Bypass check valve58 is located on bypass fuel conduit 52.

Mechanical drive 66 is connected to mechanical boost fuel pump 34 andmechanical gear-driven pump 36 by common rotational shaft 60. Controller42 is connected to starter relay 70 by first electrical connection 44A,to electric fuel pump 28 by second electrical connection 44B, and tofuel metering unit 38 by third electrical connection 44C. Power supply68 is connected to controller 42 by first power connection 74A, and tostarter relay 70 by second power connection 74B. Starter relay 70 isconnected to starter motor 72 by third power connection 74C, and toelectric pump 28 by fourth power connection 74D.

To begin operation of an APU, power supply 68 is switched on to energizecontroller 42 via first power connection 74A and starter relay 70 viasecond power connection 74B. Controller 42, now powered, sends a commandto start the APU to starter relay 70 via first electrical connection44A. Starter relay 70 either simultaneously, or in close succession,energizes starter motor 72 via third electrical connection 74C andelectric fuel pump 28 via fourth electrical connection 74D. Accordingly,starter motor 72 starts rotation of engine 40 at or near a same timethat electric fuel pump 28 begins pumping fuel along fuel supply conduit30. Fuel is more or less instantaneously provided to fuel metering unit38 a pressure and quantity sufficient to support combustor light offjust as soon as engine 40 begins rotating. Based on desired operationalspeed for engine 40, controller 42 sends signals to fuel metering unit38 via electrical connection 44C indicating the amount of fuel to flowalong fuel supply conduit 30 to engine 40.

Electric fuel pump 28 can be driven by any suitable motor including, butnot limited to, brushed DC, brushless DC, switch reluctance, corelessDC, synchronous AC, induction AC or any other type of electric motor. Ifelectric fuel pump 28 is attached to a variable speed motor, thencontroller 42 can control speed of that motor and accordingly fueloutput of electric fuel pump 28 via electrical connection 44B. Inalternative embodiments, electric fuel pump 28 is commanded directly bycontroller 42 through electrical connection 44B and independently ofstarter motor 72.

Once energized, starter motor 72 begins rotating engine 40 includingmechanical drive 66 (e.g. gear-box driven). Mechanical drive 66 operatesmechanical boost fuel pump 34 and mechanical gear-driven pump 36 viashaft 60 at a speed proportion to engine 40 speed. Once engine 40reaches about 50% of max engine speed, its operations becomeself-sustaining (i.e. mechanical boost fuel pump 34 and mechanicalgear-box driven fuel pump 36 provide sufficient fuel flow and startermotor 72 is no longer needed to assist rotation). At the point engine 40becomes self-sustaining, controller 42 commands starter relay 70 tocease operation of starter motor 72 and electric fuel pump 28 via firstelectrical connection 44A. Starter relay 70 then de-energizes bothstarter motor 72 via third power connection 74C and electric fuel pump28 via fourth power connection 74D simultaneously or in quicksuccession. Through use of starter relay 70, controller 42 activateselectric fuel pump 28 and starter motor 72 for engine starting anddeactivates starter motor 72 and electric fuel pump 28 once they are nolonger needed. The configuration depicted in FIG. 4 requires little inthe way of additional hardware or software and can be retrofitted intoexisting APU fuel systems.

FIG. 5 is a schematic diagram of fuel system 76 having shut-off valve 78and relief fuel conduit 80 for an APU. Fuel system 76 includes electricfuel pump 28, fuel supply conduit 30, fuel supply 32, mechanical boostfuel pump 34, mechanical gear-driven fuel pump 36, fuel metering unit38, engine 40, controller 42, electrical connections 44B-44D, fuelrecirculation conduit 46 having inlet 48 and outlet 50, fuel bypassconduit 52 having inlet 54 and outlet 56, bypass check valve 58,rotational shaft 60, fuel shut-off valve 78, relief fuel conduit 80having inlet 82 and outlet 84, and relief check valve 86. Fuel system 76is presented in a simplified form and can additionally include filters,heat exchangers, and/or other conventional components known in the art.Inclusion of electric fuel pump 28 allows fuel system 76 to provide fuelflow independent of engine 40 operating speed.

Fuel system 76 of FIG. 5 is similar to fuel system 62 of FIG. 3 and likenumerals indicate like components. The components (e.g. electric fuelpump 28, fuel supply conduit 30, fuel supply 32, mechanical boost fuelpump 34, mechanical gear-driven fuel pump 36, fuel metering unit 38,engine 40, controller 42, electrical connections 44, fuel recirculationconduit 46 having inlet 48 and outlet 50, fuel bypass conduit 52 havinginlet 54 and outlet 56, bypass check valve 58, rotational shaft 60) andstructure of fuel system 76 of FIG. 5 are similar to those describedabove with reference to fuel system 62 of FIG. 3. Fuel system 76 of FIG.5 differs from fuel system 62 of FIG. 3 in that fuel shut-off valve 78and relief fuel conduit 80 are added to provide additionalfunctionality. Despite the addition of shut-off valve 78 and relief fuelconduit 80, fuel system 76 of FIG. 5 can be operated in a manner similarto fuel system 62 of FIG. 3. The following discussion highlights theaddition of shut-off valve 78 and relief fuel conduit 80 in the contextof fuel system 76 of FIG. 5, but is applicable to any fuel system havingboth mechanical and electric fuel pumps (such as fuel system 64 of FIG.4, fuel system 62 of FIG. 3, and fuel system 26 of FIG. 2).

In fuel system 76, fuel supply conduit 30 fluidly connects fuel supply32 to a combustor of an engine 40. More specifically, fuel supplyconduit 30 connects fuel supply 32, mechanical boost fuel pump 34,electric fuel pump 28, mechanical gear-driven fuel pump 36, fuelmetering unit 38, fuel shut-off valve 78, and engine 40 in flow series.Along fuel supply conduit 30, fuel shut-off valve 78 is located upstreamengine 40, fuel metering unit 38 is located upstream of fuel shut-offvalve 78, mechanical gear-driven fuel pump 36 is located upstream offuel metering unit 38, electric fuel pump 28 is located upstream ofmechanical gear-driven fuel pump 36, mechanical boost fuel pump 34 islocated upstream of electric fuel pump 28, and fuel supply 32 is locatedupstream of mechanical boost fuel pump 34. Controller 42 is electricallyconnected to electric fuel pump 28 by electrical connection 44B, to fuelmetering unit 38 by electrical connection 44C, and to fuel shut-offvalve 78 by electrical connection 44D.

Fuel recirculation conduit 46 fluidly connects fuel metering unit 38 tofuel supply conduit 30 at or near mechanical gear-driven fuel pump 36.More specifically, inlet 48 of fuel recirculation conduit 46 isconnected to fuel metering unit 38, and outlet 50 of fuel recirculationconduit 46 is connected to fuel supply conduit 30 at a locationdownstream of electric fuel pump 28 and a location upstream ofmechanical gear-driven fuel pump 36. Fuel bypass conduit 52 fluidlyconnects fuel supply conduit 30 upstream of electric fuel pump 28 tofuel supply conduit 30 downstream of electric fuel pump 28. Morespecifically, inlet 54 of fuel bypass conduit 52 is connected to fuelsupply conduit 30 at a location downstream of fuel supply 32 and alocation upstream of electric fuel pump 28, and outlet 56 of fuel bypassconduit 52 is connected to fuel supply conduit 30 at a locationdownstream of electric fuel pump 28 and a location upstream ofmechanical gear-driven fuel pump 36. Bypass check valve 58 is located onbypass fuel conduit 52. Relief fuel conduit 80 fluidly connects fuelsupply conduit 30 downstream of electric fuel pump 28 to fuel supplyconduit 30 upstream of electric fuel pump 28. More specifically, inlet82 of relief fuel conduit 80 is connected to fuel supply conduit 30 at alocation downstream of electric fuel pump 28 and a location upstream ofmechanical gear-driven fuel pump 36, and outlet 84 of relief fuelconduit 80 is connected to fuel supply conduit 30 at a locationdownstream of mechanical boost fuel pump 34 and a location upstream ofelectric fuel pump 28. Relief check valve 86 is located on relief fuelconduit 80. Rotational shaft 60 connects mechanical boost fuel pump 34to mechanical gear-driven fuel pump 36.

To warm fuel prior to engine 40 starting, controller 42 commands fuelshut-off valve 78 to close and prevent fuel from flowing to engine 40.Controller 42 also commands electric pump 28 to operate and pump fuelalong fuel supply conduit 30. Fuel flows from fuel pump 28 into inlet 82of relief fuel conduit 80. Fuel flow continues along relief fuel conduit80, through open relief check valve 86 and outlet 84 of relief fuelconduit 80 back into fuel supply conduit 30. In other words, fuel isrecirculated from an outlet of electric fuel pump 28 back to an inlet ofelectric fuel pump 28. Relief check valve 86 is remains closed when apressure differential across electric pump 28 is relatively low, andopens if the pressure differential across electric pump 28 reaches apredetermine set point. Sending fuel along relief fuel conduit 80 andback through electric fuel pump 28 can be implemented to add desirableheat into fuel system 76. This pre-warming of fuel is particularlybeneficial during cold engine 40 starting. After a certain amount oftime or once the fuel reaches a certain temperature, controller 42 willcommand fuel shut-off valve 78 to open and allow fuel to flow intoengine 40. At or around the same time, a starter motor begins to rotateengine 40. At or around the same time, relief check valve 86 closes andno longer allows fuel to flow through relief fuel conduit 80, such thatfuel continues from electric fuel pump 28 through mechanical gear-drivenpump 36, fuel metering unit 38, and fuel shutoff valve 78 to engine 40.

With fuel shut-off valve 78 open, fuel flows from fuel supply 32 alongfuel supply conduit 30 to engine 40. Once engine 40 begins rotating,mechanical boost fuel pump 34 and mechanical gear-driven fuel pump 36join electric fuel pump 28 in pumping fuel along fuel supply conduit 30.As described above, mechanical boost fuel pump 34 and mechanicalgear-driven fuel pump 36 are inefficient at low engine operating speeds,and electric fuel pump 28 supplements fuel output to fuel metering unit38. Fuel metering unit 38 receives a signal from controller 42 throughelectrical connection 44C that indicates proper fuel flow to engine 40based on operational need. Fuel metering unit 38 responds to the commandfrom controller 42 by providing the proper fuel flow to engine 40 viafuel supply conduit 30 and through open fuel shut-off valve 78. Fuelmetering unit 38 can include a fuel metering valve, a delta pressurevalve, and/or a solenoid valve as known in the art. Fuel in excess ofthe amount required by engine 40 is sent from fuel metering unit 38 backto fuel supply conduit 30 along fuel recirculation conduit 46. Morespecifically, excess fuel exits fuel metering unit 38, flows throughinlet 48 of fuel recirculation conduit 46 to outlet 50 and back intofuel supply conduit 30 at a location downstream of electric fuel pump 28and upstream of mechanical gear-driven fuel pump 36.

Fuel flow output of both mechanical boost fuel pump 34 and mechanicalgear-driven fuel pump 36 is dependent upon and proportional to theoperating speed of engine 40. Accordingly, mechanical boost fuel pump 34and mechanical gear-driven fuel pump 36 are inefficient at low speedsand increasingly efficient as engine 40 speed increases. At low engineoperating speed, controller 42 commands electric fuel pump 28 to operateand supplement the inefficient pumping of mechanical boost fuel pump 34and mechanical gear-driven fuel pump 36. Since electric fuel pump 28 isdriven independently of engine 40 (e.g. by a motor), fuel flow output ofsufficient pressure and quantity is immediately available for engine 40at a lower engine speed than would be otherwise possible without assist.Once engine 40 reaches about 50% of max speed, mechanical boost fuelpump 34 and mechanical gear-driven fuel pump 36 provide more than enoughfuel flow for the needs of engine 40 (i.e. engine is self-sustaining).Controller 42 then commands electric fuel pump 28 to cease operation,thereby extending the life of electric fuel pump 28 and saving energy.At or around the same time, bypass check valve 58 on bypass fuel conduit52 will change from a closed state to an open state, thereby allowingfuel to flow along bypass fuel conduit 52. Note fuel flow along bypassfuel conduit 52 is counter to previous fuel flow along relief fuelconduit 80.

In the depicted embodiment, bypass check valve 58 is a passive pressurevalve that remains closed when the pressure differential across bypasscheck valve 58 is positive, (pressure in conduit 52 is greater than inconduit 54) and opens when a pressure differential across bypass checkvalve 58 is negative, (pressure in conduit 52 is less than in conduit54) (e.g. when electric pump 28 is turned off and not providing assist).When the pressure differential across bypass check valve 58 causesbypass check valve 58 to open, fuel will flow from mechanical boost fuelpump 34 along fuel supply conduit 30 to inlet 54 of bypass fuel conduit52, through bypass check valve 58, to outlet 56 of bypass fuel conduit52, and back into fuel supply conduit 30 at a location downstream ofelectric fuel pump 28 and upstream of mechanical gear-driven fuel pump36. In other words, fuel will bypass electric pump 28 when bypass checkvalve 58 is open. In another embodiment, fuel flows along bypass fuelconduit 52 once electric fuel pump 28 is turned off and/or when fuelflow exceeds a flow capacity of the electric fuel pump 28. Fuel system76 provides electric fuel pump 28 for supplementing fuel flow during lowspeed operation of engine 40. When electric fuel pump 28 is no longerneeded (i.e. higher speed operation of engine 40), fuel bypasseselectric fuel pump 28 and is pumped solely by mechanical boost fuel pump34 and mechanical gear-driven pump 36.

The fuel systems described above provide one or more of the followingbenefits. The electric pump provides almost instantaneous fuel flow andpressure for the combustor use at very low engine speed. The weight andsize of the mechanical pump(s) can be reduced because the typicalover-sizing for starting operation is rendered unnecessary by inclusionof the electric fuel pump. Power needed for the fuel system is reducedbecause the pumps are smaller. A reduction in power use results inbetter fuel economy. Since mechanical pump size is reduced, excess fuelis reduced thereby resulting in less fuel recirculation and less wasteheat in the fuel system. Less waste heat in the system means that thepumps run cooler and are more able to avoid cavitation. Fuel system canbe configured to operate an electric pump and recirculate fuel prior tostarting thereby warming fuel.

While the invention has been described with reference to an exemplaryembodiment(s), it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment(s) disclosed, but that theinvention will include all embodiments falling within the scope of theappended claims.

1. A fuel system for an auxiliary power unit, the fuel systemcomprising: a mechanical fuel pump for providing fuel flow to theauxiliary power unit, the mechanical fuel pump having an outputdependent upon an operational speed of the auxiliary power unit; anelectric fuel pump for providing fuel flow to the auxiliary power unit,the electric fuel pump located in series with the mechanical fuel pump;and a controller for causing the electric fuel pump to provide fuel flowduring starting of the auxiliary power unit.
 2. The fuel system of claim1, wherein the electric fuel pump is located upstream of the mechanicalfuel pump on a common fuel supply conduit.
 3. The fuel system of claim2, further comprising: a bypass conduit having a first end and a secondend, the first end fluidly connected to the fuel supply conduit upstreamof the electric fuel pump and the second end fluidly connected to thefuel supply conduit downstream of the electric fuel pump; and a bypasscheck valve located on the bypass conduit, the check valve having anopen configuration for allowing fuel to flow from the first end to thesecond end of the bypass conduit.
 4. The fuel system of claim 3, furthercomprising: a relief conduit having a first end and a second end, thefirst end fluidly connected to the fuel supply conduit downstream of theelectric fuel pump and the second end fluidly connected to the fuelsupply conduit upstream of the electric fuel pump; and a relief checkvalve located on the relief conduit, the check valve having an openconfiguration for allowing fuel to flow from the first end to the secondend of the relief conduit.
 5. The fuel system of claim 2, furthercomprising: a fuel metering unit for managing the fuel flow to theauxiliary power unit, the fuel metering unit located downstream of boththe electric fuel pump and the mechanical fuel pump on the fuel supplyconduit.
 6. The fuel system of claim 5, further comprising: arecirculation conduit having a first end and a second end, the first endconnected to the fuel metering unit and the second end connected to thefuel supply conduit at a location downstream of the electric fuel pump.7. The fuel system of claim 2, further comprising: a boost fuel pump forincreasing fuel flow to the auxiliary power unit, the boost fuel pumplocated on the fuel supply conduit upstream of the mechanical fuel pump.8. The fuel system of claim 7, wherein the boost fuel pump is connectedto the mechanical fuel pump by a rotational shaft.
 9. The fuel system ofclaim 8, wherein the electric fuel pump is located upstream of the boostfuel pump.
 10. The fuel system of claim 8, wherein electric fuel pump islocated downstream of the boost fuel pump.
 11. A fuel system for anauxiliary power unit, the fuel system comprising: a fuel supply conduitfluidly connecting a fuel reservoir to a combustor; a fuel metering unitlocated on the fuel supply conduit upstream of the combustor; amechanical gear driven pump located on the fuel supply conduit upstreamof the fuel metering unit; a mechanical boost pump located on the fuelsupply conduit upstream of the mechanical gear driven pump; an electricpump located on the fuel supply conduit upstream of the mechanical geardriven pump; and a controller for commanding the electric pump to flowfuel through the mechanical gear driven pump to the fuel metering unitduring starting of the auxiliary power unit.
 12. The fuel system ofclaim 11, further comprising: a fuel shut-off valve located on the fuelsupply conduit between the fuel metering unit and the combustor, thefuel shut-off valve having an open configuration and a closedconfiguration; and a relief conduit fluidly connecting an outlet of theelectric pump to an inlet of the electric pump, wherein fuel flows alongthe relief conduit when the fuel shut-off valve is in the closedconfiguration.
 13. The fuel system of claim 12, further comprising: astarter motor, wherein the controller is configured to command thestarter motor to start rotation of the auxiliary power unit after fuelis heated from flowing along the relief conduit.
 14. The fuel system ofclaim 11, further comprising: a starter relay connected to the electricpump; a starter motor connected to the starter relay; and a power sourceconnected to the starter relay, wherein the starter relay simultaneouslyenergizes the starter motor and the electric pump.
 15. The fuel systemof claim 11, wherein the electric pump is located between the mechanicalboost pump and the mechanical gear driven pump.
 16. The fuel system ofclaim 11, wherein the electric pump is located upstream of both themechanical boost pump and the mechanical gear driven pump.
 17. A methodof starting an auxiliary power unit, the method comprising: providingpower to a starter motor for starting rotation of the auxiliary powerunit; and providing power to an electric pump for flowing fuel to theauxiliary power unit, wherein providing power to the starter motor andproviding power to the electric pump occur simultaneously.
 18. Themethod of claim 17, further comprising: driving a mechanical pump,connected in series with the electric pump and downstream of theelectric pump, for flowing fuel to the auxiliary power unit, themechanical pump having an output dependent on operational speed of theauxiliary power unit.
 19. The method of claim 18, further comprising:terminating power to the electric pump when the output of the mechanicalpump provides sufficient fuel flow to sustain operation of the auxiliarypower unit.
 20. The method of claim 17, wherein providing power to thestarter motor and providing power to the electric pump occur through astarter relay.